The present invention is directed to digital projection systems, and more particularly to a zonal illumination system for increasing the dynamic range of digital projection systems.
A typical 3 chip color digital projection system consists of a light source, an illumination system, and a color splitting-recombining light engine. The optical function of the light engine is to split uniform illumination light into Red/Green/Blue (RGB) channels, directing each of the three channels onto an imaging device or optical panel such as an LCD (Liquid Crystal Display), DMD (Digital Micromirror Device) or LCoS (Liquid Crystal on Silicone), and then re-combining all three channels into a single illumination light beam that is projected on a screen via a projection lens.
In a LCD projector, polysilicon panels are used, one each for the red, green and blue components of the video signal. As polarized light passes through the panels (combination of polarizer, LCD panel and analyzer), individual pixels can be modulated to control the amount of light that is passed.
In a DMD imaging device, as exemplified by the Digital Light Processing (DLP™) technology of Texas Instruments, microscopic mirrors are used to electromechanically modulate light by independently flipping each mirror through a +−12 degree angle.
LCoS is a reflective technology similar to LCD except that a controlled amount of light reflects off of the front surface of the device instead of being transmitted through it.
The dynamic range of the image projected by a digital projection system is limited by the capabilities of the chosen imager. This applies equally to DLP, LCOS, LCD, as well as any other technology. In DLP™ systems, the dynamic range of a projected image is limited by the switching speed of the DMD. Grey scale aspects of the image are created using pulse width modulation (PWM) techniques. Thus, for a DMD device full white is achieved by leaving the mirrors in the on state for the duration of the frame period, full black is achieved by leaving the mirrors in the off state for the frame period, while minimal grey is achieved by having the mirrors in the on state for the shortest period of time that can be supported by the DMD (known as the “Load Time”). For the current generation of DLP™ technology this limits the dynamic range to ˜12 real bits on a 4 K DMD (assuming 24 Hz frame rate—higher rates have correspondingly fewer bits). This also assumes the minimal grey is visible against the native contrast ratio of the system. In LCOS and LCD the digital data is converted to an analog signal by a DAC (digital to analog converter). In this category of projector the dynamic range is limited by the bit depth and signal to noise ratio of the DAC.
A typical one-chip color digital projection system replaces the three imaging devices with a single imager. The light source is then modulated so that only one color of light is available at any given moment in time. This may be done in lamp based projection using a color wheel. In systems using solid state illumination (e.g. LEDs, Lasers) this may be accomplished by turning the individual colors on and off in synchronization with the content displayed on the imager. In one-chip projectors the color separating/combining prism is no longer required.
It is desirable, in digital projection systems for cinema, simulation, and other applications, to have a large dynamic range in order to create a more immersive experience.
It is therefore an objective of the present invention to increase the dynamic range of digital projection systems utilizing imaging devices such as an LCD, DMD or LCoS.